Synthesis and characterization of silica / polypyrrole nanocomposite oriented for use in organic corrosion protection coating

Nanocomposite material has a wide range of applications in various areas including metal corrosion protection. There are many methods of corrosion protection, but the simple, low cost and easy to apply method is organic protection coating. Chromat is a highly effective corrosion inhibitor pigment in organic coatings, however it is highly toxic, which causes cancer, so countries around the world have gradually eliminated chromates and research into environmental friendly - corrosion inhibitors. Corrosion inhibiting and metal protection properties of conductive polymers were first investigated by Mengoli in 1981 and DeBery in 1985, respectively. Studies have shown that polymer films formed on metal surfaces have high adhesion and good protection, however, this method has limitations on the size of the material to be protected. Therefore, recent studies have focused on the use of conductive polymers as corrosion inhibitors in organic coatings. This coating shows the advantages of conducting polymer overcomes the difficulties of film forming. These studies focus on two of the most popular and important conductive polymers: polypyrrole (PPy) and polyaniline for corrosion protection of iron / steel. Compared to polyaniline, PPy shows high electrical conductivity in both acidic and neutral environments, so it can be widely used in various fields such as energy storage devices, bio-sensors, materials photoelectric, anticorrosion coating. In addition, the synthesis of PPy films on metal substrates is easier due to the low oxidation potential of PPy. Moreover, PPy is able to stabilize better than polyaniline. However, PPy has low dispersibility, so the combination with nano additives to form nanocomposite is very interested in research. Silica nanoparticles (SiO2) have high surface area, good dispersion, ease of preparation so the use nanosilica can improve the expansion; sound insulation; flexural strength; tensile strength; and corrosion protection performance. The PPy's conductivity as well as the ability of the ion-selective redox reaction greatly depends on the nature of the polymer and the synthesis conditions. In addition, when corrosion occurs, PPy is capable of exchanging anions, so that the counter ions in the polymer also play an important role in the anticorrosion ability. Counter anions, which is small in size and highly flexible, will easily be released from the polymer network. While larger size anions can reduce bond length, leading to the increase of conductivity and solubility. Therefore, synthesis of silica/polypyrol nanocomposite and silica/polypyrol-counter anions is a promising topic, using the advantages of PPy, silica as well as anionic component. There are some studies subjecting the use of of PPy, PPy-anion, PPy/inorganic oxide. However, there is no study about silica/polypyrrole nanocomposite as well as silica/polypyrrole exchanged counter anions and its application in organic coatings for anticorrosion. Therefore, the thesis "Synthesis and characterization of silica/polypyrrole nanocomposite oriented for use in organic corrosion protection coatings" is needed, contributing to the synthesis and application of silica/polypyrrole nanocomposite in the field of corrosion protection

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1 VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY ...*** VU THI HAI VAN Project name: SYNTHESIS AND CHARACTERIZATION OF SILICA/POLYPYRROLE NANOCOMPOSITE ORIENTED FOR USE IN ORGANIC CORROSION PROTECTION COATING Major: Theoretical chemistry and Physical chemistry Code: 9440119 SUMMARY OF CHEMICAL DOCTORAL THESIS Hanoi – 2018 2 The thesis was completed at: Graduate University of Science and Technology - Vietnam Academy of Science and Technology Scientific Supervisors: 1. Assoc. Prof. Dr., To Thi Xuan Hang 2. Assoc. Prof. Dr. Dinh Thi Mai Thanh Referee 1: Referee 2: Referee 3: The thesis will be defended in front of doctoral thesis judgement, held at ., .. The thesis can be found at: - Library of Graduate University of Science and Technology - National library of Vietnam 3 A. INTRODUCTION 1. The necessity of the research Nanocomposite material has a wide range of applications in various areas including metal corrosion protection. There are many methods of corrosion protection, but the simple, low cost and easy to apply method is organic protection coating. Chromat is a highly effective corrosion inhibitor pigment in organic coatings, however it is highly toxic, which causes cancer, so countries around the world have gradually eliminated chromates and research into environmental friendly - corrosion inhibitors. Corrosion inhibiting and metal protection properties of conductive polymers were first investigated by Mengoli in 1981 and DeBery in 1985, respectively. Studies have shown that polymer films formed on metal surfaces have high adhesion and good protection, however, this method has limitations on the size of the material to be protected. Therefore, recent studies have focused on the use of conductive polymers as corrosion inhibitors in organic coatings. This coating shows the advantages of conducting polymer overcomes the difficulties of film forming. These studies focus on two of the most popular and important conductive polymers: polypyrrole (PPy) and polyaniline for corrosion protection of iron / steel. Compared to polyaniline, PPy shows high electrical conductivity in both acidic and neutral environments, so it can be widely used in various fields such as energy storage devices, bio-sensors, materials photoelectric, anti- corrosion coating. In addition, the synthesis of PPy films on metal substrates is easier due to the low oxidation potential of PPy. Moreover, PPy is able to stabilize better than polyaniline. However, PPy has low dispersibility, so the combination with nano additives to form nanocomposite is very interested in research. Silica nanoparticles (SiO2) have high surface area, good dispersion, ease of preparation so the use nanosilica can improve the expansion; sound insulation; flexural strength; tensile strength; and corrosion protection performance. The PPy's conductivity as well as the ability of the ion-selective redox reaction greatly depends on the nature of the polymer and the synthesis conditions. In addition, when corrosion occurs, PPy is capable of exchanging anions, so that the counter ions in the polymer also play an important role in the anticorrosion ability. Counter anions, which is small in size and highly flexible, will easily be released from the polymer network. While larger size anions can reduce bond length, leading to the increase of conductivity and solubility. Therefore, synthesis of silica/polypyrol nanocomposite and silica/polypyrol-counter anions is a promising topic, using the advantages of PPy, silica as well as anionic component. There are some studies subjecting the use of of PPy, PPy-anion, PPy/inorganic oxide. However, there is no study about silica/polypyrrole nanocomposite as well as silica/polypyrrole exchanged counter anions and its application in organic coatings for anticorrosion. Therefore, the thesis "Synthesis and characterization of silica/polypyrrole nanocomposite oriented for use in organic corrosion protection coatings" is needed, contributing to the synthesis and application of silica/polypyrrole nanocomposite in the field of corrosion protection. 2. The main contents and objectives of the thesis - Investigation of the synthesis parameters of silica/polypyrrole and silica/polypyrrole-doped anions nanocomposite by in-situ method. - Characterization and corrosion inhibitor abilities of silica/polypyrrole nanocomposites for carbon steel. - Evaluation of corrosion protection for carbon steel of polyvinylbutyral and epoxy coatings containing silica/polypyrrole-doped anions. 4 3. The scientific significance, practicality and new contributions of the thesis - Silica/polypyrrole nanocomposites were synthesized by in-situ method in the presence of doped anions, such as: dodecyl sulfate, benzoate and oxalate. The synthesize nanocomposites have spherical structure, diameter in the range of 50-150 nm. Nanocomposite contains the oxalate anion showed the best inhibitor ability in polivinylbutyral coatings. - The potential application of silica/polypyrrole-doped oxalate nanocomposite in epoxy coatings has been evaluated for corrosion protection. The results were obtained by electrochemical methods showed that silica/polypyrrole-doped oxalate nanocomposite significantly improved corrosion resistance of epoxy coating. The results open up the prospect of using silica/polypyrrole-doped oxalate nanocomposite as a corrosion inhibitor in organic coatings. 4. Structure of the thesis The thesis includes 127 pages: introduction (3 pages), the overview (35 pages), experimental (13 pages), results and discussions (60 pages), conclusion (1 page) , new contributions of the thesis (1 page), list of published scientific works (1 page), 9 tables, 63 images and graphs, 141 references. B. CONTENT OF THE THESIS CHAPTER I. OVERVIEW The thesis has summarized literature over the world about synthesis of silica, polypyrrole, silica/polypyrrole composites and its application, special in anticorrosion. CHAPTER II. EXPERIMENTAL 2.1 Materials - Pyrrole, C4H5N, (97 %, Germany); TEOS, Si(OC2H5)4, (South Korea); PVB, (C8H14O2)n, (Japan). - HCl, FeCl3, Na2C2O4, CH3(CH2)11OSO3Na, NaC6H5CO2, C3H6O, CH4O (China). - Epoxy bisphenol A, Epotec YD011-X75 and Polyamide 307D-60 (South Korea). 2.2 Synthesis of silica/polypyrrole nanocomposites 2.2.1 Silica TEOS was dropped slowly into 140 ml HCl solution with pH = 1. The mixture was stirred for 24 hours at room temperature, and then was heated at 80oC during 24 hours. The precipitate was washed with distilled water to pH = 7 and dried at 80oC for 24 hours in a vacuum oven. 2.2.2 Silica/polypyrrole nanocomposites Prepared three solutions: - Solution 1: SiO2 were dispersed in 40 ml H2O or C2H5OH by ultra-sonic in 30 minutes. - Solution 2: 1 mmol pyrrole were dispersed in 20 ml H2O. - Solution 3: 0.05 mol FeCl3.6H2O were dissolved in 40 ml H2O or C2H5OH Solution 2 was dropped slowly into solution 1, stirred for 1 hour. Then solution 3 was dropped slowly into above mixture, stirred for 24 hours. The mixture was filtered and washed 5 times with distilled water and once with mixture of methanol and acetone to remove unwanted products. The precipitate was dried at 80oC in 24 hours in vacuum oven. To synthesis SiO2/PPy-doped anions, follow the same process, only additional of 2.5 mmol NaC2O4 (Ox) or NaC12H25SO4 (DoS) or C7H5NaO2 (Bz) in solution 2. 2.2.3 Synthesis polyvinylbutyral coatings containing SiO2/PPy nanocomposites 5 Step 1: Carbon steel sheets were used as substrate (10×5×0.2 cm). The sheets were cleaned with soap, distilled water and ethanol, dried and marked. Then the sheets were polished with abrasive paper 600 grades, washed by distilled water, ethanol and dried. Step 2: PVB solution was prepared by dissolving 10 wt% of PVB in mixture of propanol and ethanol (ratio 1:1). The SiO2/PPy nanocomposites were dispered into PVB solution by continuous magnetic stirring and sonication for 4 hours. Step 3: The liquid paints were deposited on the bare steel using a spin-coater at rotating speeds up to 600 rpm. Finally, all samples were dried at ambient temperature for 7 days. The dry films thickness is about 11±2 µm (measured by Minitest 600 Erichen). 2.2.4 Synthesis epoxy coatings containing SiO2/PPy nanocomposites The synthesize process is similar, epoxy solutions were prepared by dispersed 5 wt% SiO2/PPy nanocomposites into epoxy and xylene by magnetic stirring. The epoxy coatings containing SP, SPO1, SPO2 and SPO3 were labeled as ESP, ESPO1, ESPO2 and ESPO3, respectively. The rotating speed is 1000 rpm due to the high viscosity of epoxy solutions. The dry films thickness is about 25±2 µm (measured by Minitest 600 Erichen). 2.3 Methods - IR, SEM, TEM, EDX, UV-Vis were measured at Institute for Tropical technology, National Institute Of Hygiene And Epidemiology and Future Industry Institute. - TGA were performed with a heating rate of 10oC per minute, from 25-850oC, in air, at Future Industry Institute. - X-ray diffractometer were carried out with scanning rate 0.03° per second and 2 theta (2θ) angle ranging from 10° to 80° at current 40.0 mA and voltage 40.0 kV, at Future Industry Institute. - XPS were measured at Future Industry Institute using X-radiations with Al at 15 kV- 15 mA. - The conductivities were measured by cyclic voltammetry method through the two-point-electrode without electrolyte with sample thickness is 1 cm and sample area is 1 cm2. - Open circuit potential and electrochemical impedance spectra were measured at Institute for Tropical technology. - Salt spray test was carried out followed by ASTM B117 standard at Institute for Frontier Materials. CHAPTER III. RESULTS AND DISCUSSIONS 3.1 Synthesis and characterization of SiO2/PPy nanocomposties 3.1.1 Effect of synthesis solution Synthesis solution plays an important role in dispersive ability, morphology and characterization of SiO2/PPy nanocomposites. There were some studies reported that the presence of ancol can improve the dispersion and modify surface characteristic of silica. Therefore, SiO2/PPy nanocomposites were synthesized in solution containing water, mixture of ethanol: water = 2:3 and mixture of ethanol : water = 4:1, labeled as SiO2/PPy-W, SiO2/PPy-EW and SiO2/PPy-E, respectively. IR spectra (Figure 3.1) of SiO2/PPy-W, SiO2/PPy-E and SiO2/PPy-EW showed similar trend, included characteristic bands of SiO2 (~471, 794 and 1080 cm -1) and PPy (~1530, 1450, 1405 and 1050 cm-1). EDX results of SiO2/PPy-W, SiO2/PPy-E and SiO2/PPy-EW are shown in figure 3.2. The spectra show pic of silicon and oxygen, which is from silica; carbon, nitrogen and chloride, which is 6 from polypyrrole. Weight percentages of silicon increase from 20.18 to 21.07 and 22.08% with SiO2/PPy-W, SiO2/PPy-E and SiO2/PPy-EW, respectively. Figure 3.1. FT-IR spectra of SiO2, PPy and SiO2/PPy nanocomposites Figure 3.2. EDX diagrams of SiO2, PPy and SiO2/PPy nanocomposites SEM photographs of synthesized SiO2 and SiO2/PPy nanocomposites are shown in figure 3.3. The synthesized nanocomposites have similar morphology with spherical shape. Diameter of nanocomposites is higher than silica. It can be explained by the deposition of pyrrole on the silica surface, the polymerization of pyrrole in the presence of oxidation agent. Figure 3.3. SEM photographs of SiO2 (a), SiO2/PPy-W (b), SiO2/PPy-EW (c) and SiO2/PPy-E (d) Figure 3.5 shows UV-Vis spectra of SiO2, PPy SiO2/PPy-W, SiO2/PPy-E and SiO2/PPy-EW. Characteristic peak of silica is observed at 300 nm. In the case of PPy, there are two main peak, at 400-450 nm and broad peak at 900-1100 nm. The first peak at low wavelength is presented for band gap of π-π* bond. In the other hand, this peak also confirms the bipolarons state of PPy. Peak at higher wavelength is characterized for conductive electron. In comparison between spectra of PPy and nanocompositess, there is the change of peak position to higher wavelength zone. This result indicated the longer conjugated bond, corresponding with the higher conductivity. 7 Figure 3.5. UV-Vis spectra of samples Figure 3.6. CV diagram of samples The electrical conductivities of samples were determined through CV-diagrams from figure 3.6. PPy has the highest conductivity, 0.432 S.cm-1. The conductivities of nanocomposites synthesized in water, ethanol:water = 2:3 and ethanol:water = 4:1 is 0.19, 0.14 and 0.11 S.cm-1, respectively. It can be explained by the insulation of silica. Figure 3.7 showed the survey scans of PPy, SiO2/PPy-W, SiO2/PPy-EW and SiO2/PPy-E. PPy spectra showed characteristic peak of carbon C1s, nitrogen N1s and clo Cl2p, in agreement with EDX results. In comparison with PPy, XPS spectra of nanocomposites have two more peak, at 101.9 eV and 531.5 eV, represented for silicon Si2p and oxygen O1s. These results indicated the presence of silica in nanocomposites. With PPy, the high resolution spectra included four components (figure 3.8). At the lowest bonding energy and highest intensity, the main peak at 285.1 eV, represented for C-C bond between Cα and Cβ in pyrrole ring. Peak at 286.2 eV; 287.8 eV and 290.4 eV indicated PPy at doped state. Peak presented for C=N and =C-NH•+ (polaron) bond is observed at 286.2 eV. The peak at 287.8 eV is assigned to –C=N+ bond of bipolaron PPy. Figure 3.7. XPS spectra of PPy, SiO2/PPy-W, SiO2/PPy-EW and SiO2/PPy-E With N (1s) high resolution spectrum showed three components (figure 3.9). The signal at 399.6 eV was assigned to the –NH group of pyrrole ring. At higher bonding energy, there were two 8 peak which assigned to pyrrole at doped state. Peaks at 400.5 eV and 402.4 eV were assigned to NH•+ of polaron PPy and =NH+ of bipolaron PPy, respectively. Figure 3.8. High resolution C1s and N1s of PPy Figure 3.9. High resolution C1s and N1s of SiO2/PPy-W The high resolution C1s and N1s spectra of SiO2/PPy-W, SiO2/PPy-EW and SiO2/PPy-E nanocomposite were shown in figure 3.9, 3.10 and 3.11, respectively. All the spectra showed similar trend with PPy. However, the bonding energy of nanocomposites is lower than that of PPy. This result indicated the decrease of conjugated bond length, according to the lower conductivity, in agreement with conductivity measurement. From the analysis of XPS spectra, the weight percentages of each element and oxidation state of nitrogen were listed in table 3.3. The results indicated that when change the synthesis solution, weight percentage of element insignificant changed. Percentages of nitrogen at neutralize and polaron state were higher than that of nanocomposites. It showed the higher oxidative ability. Therfore, the percentages of nitrogen at bipolaron state are higher, the lower conductivities. Figure 3.10. High resolution C1s and N1s of SiO2/PPy-EW Figure 3.11. High resolution C1s and N1s of SiO2/PPy-E Bảng 3.3. Analysis parameter from XPS spectra Sample Weigh percentage (%) Oxidation state (%) C N O Si Cl -N+= -NH- -N+ PPy 74,5 23,6 - - 1,9 0,08 0,65 0,27 SiO2/PPy-W 35,7 7,8 32,6 22,4 1,5 0,17 0,58 0,25 SiO2/PPy-EW 35,4 7,5 32,5 23,3 1,3 0,21 0,55 0,24 SiO2/PPy-E 34,5 7,7 32,6 23,8 1,4 0,24 0,51 0,25 3.1.1. Effect of pyrrole/silica ratio The quantities of silica showed important affect to the formation of nanocomposites. Therefore, in this study, SiO2/PPy nanocomposites were synthesized at constant quantity of PPy and silica changed from 2,5 mmol (SP1); 5 mmol (SP2); 7,5 mmol (SP3) to 10 mmol (SP4). The ratio of pyrrole/silica changed from 0.4, 0.2, 0.13 to 0.1, respectively. 9 Figure 3.12. IR spectra of SiO2, PPy, SP1, SP2, SP3 and SP4 Figure 3.13. EDX spectra of SiO2, PPy, SP1, SP2, SP3 and SP4 IR spectra of nanocomposites showed characteristic peak of silica (1080, 793 and 471 cm-1) and polypyrolle (1530 and 1450 cm-1) (figure 3.12). Its indicated the presence of silica in nanocomposites. With SP1, the quantity of silica is low, therefore, the characteristic peaks had lower intensity when peak of PPy had higher intensity. When silica percentage is increase, from SP2 to SP4, IR spectra showed strong peak of silica at 1080 cm-1. EDX results showed four main elements in nanocomposites: carbon, nitrogen, oxygen and silicon (figure 3.13). When the quantity of silica in synthesized solution increase, the weight percentage of silicon in nanocomposites increase, from 20.48 to 21.19, 25.03 and 28.14%, in SP1,SP2, SP3 and SP4, respectively. SEM photographs of silica, SP1, SP2, SP3 and SP4 were shown in figure 3.14. All the samples had spherical shapes. When forming nanocomposites, diameter of sample was increase. Moreover, when the quantity of silica increased, the particles sizes also increased. It might due to the polymerization of PPy, cover silica shell. Figure 3.14. SEM photographs of SiO2 (a), SP1 (b), SP2 (c), SP3 (d) and SP4 (e) 10 Figure 3.15. TGA diagrams of PPy, SP1, SP2, SP3 and SP4 Figure 3.15 showed TGA diagrams of samples. With SP1, SP2, SP3 and SP4, TGA diagrams had same trend, the weight loss was 48.5, 42.2, 38.1 and 32%, respectively. TGA diagrams consisted of two stages: an initial weight loss at less than 100oC due to the loss of water absorption in the surface. The second loss from 100-650oC might due to the degradation of polypyrrole blackbone and the decomposition of oxidation agent. However, the total weight loss of nanocomposites was lower than that of PPy. It can be explained by the high thermal resistance of silica. Calculated from TGA results, weight percentage of silica in SP1, SP2, SP3 and SP4 is 51, 57, 61 and 67%. 3.1.2. Electrochemical characteristic of SiO2/PPy nanocomposites 3.1.3.1. Inhibitive ability in NaCl 3% solution Figure 3.16 showed the open circuit potential of carbon steel immerse in NaCl 3% include and not include 3 g/L SP1, SP2, SP3 and SP4 nanocomposites after 36 hours. Initially, for bare steel, OCP value reached -0.6 VSCE, then decreased over time. After 20 hours of immersion, OCP value is -0.7 VSCE, and kept stably. The decrease of OCP can be explained by the erosion formation. After 36 hours of immersion, the OCP value of bare steel was -0.7 VSCE, reaching the corrosion potential of steel. In the case of SP1, SP2, SP3 and SP4, OCP varied with the same trend. In the beginning, it reached -0.32, -0.32, -0.37 and -0.40 VSCE, respectively. These results showed that SiO2/PPy nanocomposites can shift the OCP of steel to passive region, which is demonstrated the role of anodic inhibitor. Over time, the value of OCP dropped toward negative value, however, always positive than that of bare steel. Therefore, SiO2/PPy showed good inhibitive ability, but it decreased overtime due to the erosion of corrosive agents. After 36 hours of immersion, the OCP of SP1, SP2, SP3 and SP4 were -0.63, -0.64, -0.68 and -0.68 VSCE, respectively. 11 Figure 3.16. OCP variation over time of carbon steel in 3%NaCl solution include and not include 3g/L SP1 (b), SP2 (c), SP3 (d), SP4 (e) after 36 hours of immersion. 3.1.3.2. Corrosion protection for carbon steel of PVB film containing SiO2/PPy nanocomposites PVB is an organic coating, which is easy for synthesis, non-toxic and short tested time. Therefore, PVB film was used to investigate the inhibitive ability of SiO2/PPy nanocomposites. A, Open circuit potential Figure 3.19 showed the OCP variation of carbon coated with PVB and PVB containing 10 wt% nanocomposites. With carbon steel coated with PVB, in the beginning, OCP value was -0.4 VSCE. This result confirmed the good barrier p
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